Re: [Qemu-devel] [RFC 2/2] block/file-posix: verify page cache is not used

[Kevin Wolf <kwolf@redhat.com>]

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Am 20.04.2018 um 05:02 hat Stefan Hajnoczi geschrieben:
> On Thu, Apr 19, 2018 at 10:05:47AM +0100, Dr. David Alan Gilbert wrote:
> > * Stefan Hajnoczi (stefanha@redhat.com) wrote:
> > > This commit is for debugging only.  Do not merge it.
> > > 
> > > mincore(2) checks whether pages are resident.  Use it to verify that
> > > page cache has been dropped.
> > > 
> > > You can trigger a verification failure by mmapping the image file from
> > > another process and loading a byte from a page so that it becomes
> > > resident.  bdrv_co_invalidate_cache() will fail while the process is
> > > alive.
> > 
> > It doesn't seem a bad diagnostic to keep in (with a switch to activate)
> > for when we're faced with some weird corruption on some weird storage
> > system.
> 
> Okay.  It's very slow to mmap an entire image file and query mincore(2)
> so it needs to be off by default.

Also, having it enabled breaks localhost migration at least on tmpfs
(which was what I tried out first).

I wonder if the kernel would add some way to query whether the "advice"
was actually acted upon if we asked. Either with a new function that
returns an error if not everything is dropped (basically
.bdrv_invalidate_cache on the kernel level), or a function that just
queries if any page is allocated (or maybe the address of the first
allocated page in a given range) without having to use mincore() and
iterating over all the pages in userspace.

Kevin

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